1. Field of the Invention
The invention relates to a storage apparatus for feedback controlling a moving position of a carriage so as to set a positional deviation amount of a head from a track center to zero and, more particularly, to a storage apparatus for obtaining a control signal to suppress a positional deviation of a repetitive disturbance such as a medium eccentricity by a learning control and performing a feed-forward control.
2. Description of the Related Arts
In a conventional optical disk apparatus, to raise track-following performance of a laser beam to medium tracks, there is used a head mechanism of a double driving type comprising: a carriage actuator for a seek control (also referred to as a coarse control) for moving a carriage supported by a bearing unit for a guide rail fixedly arranged; and a tracking actuator for a track-following control (also referred to as a fine control) for moving the laser beam in the direction which transverses the tracks by the driving of an objective lens mounted on the carriage. In recent years, there has also widely been spread a head mechanism of a single driving type comprising only the carriage actuator by omitting the tracking actuator in order to reduce the costs of the apparatus. In the head mechanism of the single driving type, a slide bearing is employed replacing the ball bearing, thereby reducing the number of parts and the costs. However, in case of constructing the head mechanism as a mechanism of the single driving type comprising only the carriage actuator and, further, removing the ball bearing from the bearing unit of the carriage, a positioning control of the laser beam for the track center based on a tracking error signal is strongly influenced by a Coulomb friction which the carriage bearing unit has.
FIG. 1 shows characteristics of the Coulomb friction in the carriage of the single driving type. Each of a moving velocity V and a frictional force F has a plus or minus value according to the moving direction of the carriage. A case where the moving velocity V of the carriage changes from the minus value to the plus value will now be considered. While the carriage is moving at the moving velocity V having the minus value, an almost constant kinetic frictional force F1 of a plus value is generated against the moving velocity. When the moving velocity V of the carriage for the guide rail is equal to 0 and, subsequently, the carriage starts to move in the opposite direction, a driving force canceling a static frictional fore -F2 is needed and, after moving, the driving force should include a force canceling an almost constant kinetic frictional force -F1. At the time of the reversal of the moving velocity of the carriage as mentioned above, a steep force change of the frictional force acts as a disturbance on a control system. To sufficiently compensate the disturbance, a feedback control system with a high bandwidth is generally necessary. The reversal of the moving velocity of the carriage occurs, for example, in a track-following control to compensate for a repetitive positional deviation of the track due to an eccentricity of the medium. That is, when the carriage is controlled so as to trace the medium eccentricity, the motion of the carriage for the guide rail becomes a reciprocating motion synchronized with an eccentricity period. Therefore, the moving velocity of the carriage is reversed at least twice for one rotation of the medium and is subjected to the disturbance by the steep force change of the frictional force each time.
FIG. 2 shows a simulation result of a tracking error signal TES for a rotational period (time) when an on-track control is performed by a feedback control system to a head mechanism of a single driving type. In such a simulation, a track pitch is set to 1.1 xcexcm, a rotational speed of the disk is set to 3600 rpm, and a coefficient xcexc of friction is set to 0.3. A band of the feedback control system is set to 1.5 kHz in consideration of a high-order resonance having a higher resonance at about 15 kHz of the actual head mechanism. A waveform 200 relates to a case where a peak-to-peak amount of eccentricity is assumed to be 50 xcexcm. A waveform 202 relates to a case where a peak-to-peak amount of eccentricity is assumed to be 20 xcexcm. A waveform 204 relates to a case where a peak-to-peak amount of eccentricity is assumed to be 10 xcexcm. With respect to any of the waveforms 200, 202, and 204 as well, the eccentricity disturbance cannot be sufficiently suppressed due to deterioration of low band error compressing performance and reduction of a control band of the feedback control system due to the single driving of the head mechanism. The waveforms are also influenced by the steep change of the frictional disturbance occurring at a point when the moving velocity is equal to 0 when the carriage is allowed to trace the eccentricity, so that large peak-like tracking errors 206-1, 206-2, 206-3, and 206-4 occur. If a Coulomb friction Ffric in association with the movement of the carriage is simply expressed by omitting a static friction, it is modeled by the following equation.                               F          fric                =                  {                                                                                                                -                      μ                                        ⁢                                          xe2x80x83                                        ⁢                    mg                                    ,                                                            ϰ                      .                                        ≥                    0                                                                                                                                            μ                    ⁢                                          xe2x80x83                                        ⁢                    mg                                    ,                                      ϰ                     less than                     0                                                                                                          (        1        )            
As will be obviously understood from such a model, a cause of difficulty of the compensation by the feedback control is considered because the sign of the Coulomb friction Ffric suddenly changes, for example, from the minus value to the plus value at the time of reversal of a velocity {dot over ("khgr")} of the carriage for the guide tail.
Although a method of raising the band of the feedback control system is generally considered as a method of compensating for such a steep frictional disturbance, there is a limitation due to the existence of the high-order mechanical resonance near 15 kHz. Further, since the track-following control is performed by the carriage driving and the driving by the lens actuator is omitted, it is difficult to sufficiently raise the control band for positioning.
According to the invention, there is provided a storage apparatus in which by combining a feedback control system and a learning control system, a steep frictional disturbance due to a medium eccentricity is certainly compensated for, and a tracking error is reduced.
First, a storage apparatus of the invention, for example, an optical storage apparatus comprises: a head having a carriage for moving an irradiating position of a laser beam onto an arbitrary track position on a medium; a position signal detecting unit (tracking error detecting unit) for detecting and generating a position signal (tracking error signal) TES according to a positional deviation amount in which a predetermined position of a track on the medium is used as a reference on the basis of the light derived from the medium in accordance with an irradiation of the laser beam; a feedback calculating unit for inputting the position signal TES and calculating a control signal (control current) IFB to move the carriage of the head so as to set the positional deviation amount to zero; and a driving unit (VCM) for driving the carriage of the head so that the irradiating position of the laser beam traces the track on the basis of the control signal IFB of the feedback calculating unit. With respect to such a storage apparatus, the invention is characterized by comprising a learning control unit for getting an unknown function for one medium rotation to set the positional deviation amount for the repetitive disturbance to zero as an approximated function which was approximately presumed by a learning algorithm and storing it. More specifically speaking, according to the learning control unit, the unknown function for one medium rotation period to set the positional deviation amount for the repetitive disturbance such as a medium eccentricity synchronized with the medium rotation to zero is obtained by a learning algorithm as an approximated function which was approximately presumed by a set of heights of N rectangular functions which a time width of each rectangular function is obtained by dividing the time for one medium rotation period into N intervals and stored. According to the learning control unit, even if it takes a slightly long time to converge the learning result due to a low learning gain, a compensation signal of a steep frictional disturbance with a high bandwidth in association with the reverse in the carriage moving direction can be also included in the learning result that is finally obtained. By adding such a learning control signal to a feedback control signal as a feed-forward compensation signal, the steep frictional disturbance can be almost cancelled. Since the single driving type carriage is used, there is a limitation due to the existence of the high-order mechanical resonance. Even if the control band is low, a tracking error for the eccentricity of the medium is remarkably reduced and the precision of the on-track control can be improved.
The learning control unit of the invention is provided between the feedback calculating unit and the driving unit. Assuming that the time for one medium rotation period is set to TL, an unknown drive current function Irepeat(t) (where, 0xe2x89xa6t less than TL; TL denotes one medium rotation period) which is repeated for a period of time from a start time t=0 for one medium rotation period to an end time t=TL is obtained by a learning algorithm as an approximated function I{circumflex over ( )}repeat(t) (where, 0xe2x89xa6t less than TL; TL denotes the one medium rotation period) which is approximately presumed by a set of heights of N rectangular functions indexed from 0 to (Nxe2x88x921), obtained by dividing the time TL for one medium rotation period into N intervals and stored. Although the approximated function is expressed by
I{circumflex over ( )}repeat
it is expressed as xe2x80x9cI{circumflex over ( )}repeatxe2x80x9d in the specification. This expression is also similarly applied to an approximated function xe2x80x9cTES{circumflex over ( )}repeatxe2x80x9d.
When the learning control unit is provided between the feedback calculating unit and the driving unit as mentioned above, since the learning of the drive current of the feedback control system is performed, a learning result of small noises is obtained. The learning result can be directly used as a feed-forward current at the time of a seek control, a kickback, or the like after the learning. The control is simpler and more certain because the conversion of the learning result is unnecessary.
The learning control unit comprises a memory, a sampling unit, an approximated function calculating unit, and a feed-forward output unit. The memory has a plurality of memory cells to store the height Ci of each rectangular function of the approximated function I{circumflex over ( )}repeat(t). The sampling unit samples the control signal IFB which is outputted from the feedback calculating unit. The approximated function calculating unit obtains the height Ci of each, rectangular function of the approximated function I{circumflex over ( )}repeat(t) stored in each memory cell of the memory by the following learning law.
Ci=Klearnxc3x97IFB
where, i denotes the index number of the rectangular function which is decided by time t and 0xe2x89xa6ixe2x89xa6(Nxe2x88x921);
for example, i=floor(t/T), where T=TL/N
on the basis of the control signal IFB sampled by a sampling unit and a predetermined learning gain Klearn and updates the height Ci.
A feed-forward output unit (FF output unit) reads out the height Ci, as a learning control signal, of each rectangular function of the approximated function I{circumflex over ( )}repeat(t) stored in the memory cell of the memory synchronously with the divisional period T of the medium rotation, adds it to the control signal IFB from the feedback calculating unit, and supplies a drive signal IVCM to the driving unit.
In more detail, the sampling unit samples the control signal IFB at a predetermined period Tsample shorter than or equal to the divisional period T, and the approximated function calculating unit obtains the height Ci of each rectangular function of the approximated function I{circumflex over ( )}repeat(t) stored in each memory cell of the memory by the following equation
Cinew=Cilast+Klearnxc2x7Tsamplexc2x7IFB(t)
where, i denotes the number of the interval which is decided by a time t and 0xe2x89xa6ixe2x89xa6(Nxe2x88x921);
for example, i=floor(t/T)
on the basis of the control signal IFB sampled by the sampling unit and a predetermined learning gain Klearn and updates the height Ci. Cilast denotes a value of Ci before the updating and Cinew denotes a value of Ci after the updating. In the equation, for example, Ci to be updated at present time t is selected on the basis of a calculation result of i by i=floor(t/T). An integrating arithmetic operation which inputs IFB(t) is performed to the Ci value (Cilast) of one sample before (before Tsample time), thereby obtaining an updating result (Cinew) of the Ci value at the present time t. The height Ci of rectangular function having an index number other than i calculated by i=floor(t/T) is not updated (namely, in this case, Cinew=Cilast). The above processes are summarized as follows.   "AutoLeftMatch"      {                                                                      C                i                new                            =                                                C                  i                  last                                +                                                      K                    learn                                    ·                                      T                    sample                                    ·                                                            I                      FB                                        ⁡                                          (                      t                      )                                                                                            ,                                                            for            ⁢                          xe2x80x83                        ⁢            i            ⁢                          xe2x80x83                        ⁢            which            ⁢                          xe2x80x83                        ⁢            is            ⁢                          xe2x80x83                        ⁢            calculated            ⁢                          xe2x80x83                        ⁢            by                                                            i            =                          floor              ⁢                              xe2x80x83                            ⁢                              (                                  t                  /                  T                                )                                                                                                                    C                i                new                            =                              C                i                last                                      ,                          for              ⁢                              xe2x80x83                            ⁢              the              ⁢                              xe2x80x83                            ⁢              other              ⁢                              xe2x80x83                            ⁢              i                                          
Further, synchronously with the medium rotation, a feed-forward output unit reads out the height Ci of each rectangular function of the approximated function I{circumflex over ( )}repeat(t) stored in the memory cell of the memory synchronously with the sampling period Tsample , adds it to the control signal IFB from the feedback calculating unit, and supplies a drive signal IVCM to the driving unit. The feed-forward output unit reads out the value of the approximated function I{circumflex over ( )}repeat(t) stored in each memory cell of the memory corresponding to the time that is advanced by a predetermined time xcex94tlead and outputs it. The learning control unit repeats the learning while feed-forward outputting the learning result at this time point. It this case, there is a time delay such as a phase delay or the like in the feedback control system. Unless it is compensated, the control becomes unstable. Therefore, with respect to the latest learning result I{circumflex over ( )}repeat(t) at that time point, the value corresponding to the time that is advanced from the present time by the predetermined time xcex94tlead is read out and outputted, so that the learning can be performed in a state where the control system is stable.
In another embodiment of the invention, the learning control unit is provided between the position signal detecting unit and the feedback calculating unit. Assuming that the time corresponding to one medium rotation is equal to TL, an unknown position function TESrepeat(t) (where, 0xe2x89xa6t less than TL; TL denotes the period for one medium rotation period) which repeats for a period of time from the start time t=0 to the end time t=TL of the one medium rotation is obtained by a learning algorithm as an approximated function TES{circumflex over ( )}repeat(t) (where, 0xe2x89xa6t less than TL; TL denotes the period for one medium rotation period) which is approximately presumed by the height Ci (where, i is the index number; 0xe2x89xa6ixe2x89xa6(Nxe2x88x921)) of each rectangular function having the index number i obtained by dividing the time TL for one medium rotation period into N intervals and is stored. The learning control unit has a memory, a sampling unit, an approximated function calculating unit, and a feed-forward output unit. The memory has a plurality of memory cells to store the height Ci of each: rectangular function of the approximated function TES{circumflex over ( )}repeat(t). The sampling unit samples the position signal TES which is outputted from the position signal detecting unit. The approximated function calculating unit obtains the height Ci of each rectangular function of the approximated function TES{circumflex over ( )}repeat(t) stored in each memory cell of the memory by the following equation
{dot over (C)}i=Klearnxc2x7TES(t)
where, i denotes the index number of the interval which is decided by time t and 0xe2x89xa6ixe2x89xa6(Nxe2x88x921);
for example, i=floor(t/T), T=TL/N
on the basis of the position signal TES sampled by the sampling unit and the predetermined learning gain Klearn and updates the height Ci. Synchronously with the medium rotation, the feed-forward output unit reads out the height Ci of each rectangular function of the approximated function TES{circumflex over ( )}repeat(t) stored in the memory cell synchronously with the divisional period T of the medium rotation, adds it to the position signal TES from the position signal detecting unit, and supplies a feedback signal TESFB to the feedback calculating unit. Explaining in more detail, the sampling unit samples the position signal TES at a predetermined period Tsample shorter than or equal to the divisional period T. The approximated function calculating unit obtains the height Ci of each rectangular function of the approximated function TES{circumflex over ( )}repeat(t) stored in each memory cell of the memory by the following equation
xe2x80x83Cinew=Cilast+Klearnxc2x7Tsamplexc2x7TES(t)
where, i denotes the number of the interval which is decided by time t and 0xe2x89xa6ixe2x89xa6(Nxe2x88x921);
for example, i=floor(t/T)
on the basis of the control signal IFB sampled by the sampling unit and a predetermined learning gain Klearn and updates the height Ci. Cilast denotes a value of Ci before the updating and Cinew denotes a value of Ci after the updating. In the equation, for example, Ci to be updated at present time t is selected on the basis of a calculation result of i by i=floor(t/T). An integrating arithmetic operation which inputs TES(t) is performed to the Ci value (Cilast) of one sample before (before Tsample time), thereby obtaining an updating result (Cinew) of the Ci value at the present time t. The height Ci of rectangular function having an index number other than i calculated by i=floor(t/T) is not updated (namely, in this case, Cinew=Cilast). The above processes are summarized as follows.   "AutoLeftMatch"      {                                                                      C                i                new                            =                                                C                  i                  last                                +                                                      K                    learn                                    ·                                      T                    sample                                    ·                                      TES                    ⁡                                          (                      t                      )                                                                                            ,                                                            for            ⁢                          xe2x80x83                        ⁢            i            ⁢                          xe2x80x83                        ⁢            which            ⁢                          xe2x80x83                        ⁢            is            ⁢                          xe2x80x83                        ⁢            calculated            ⁢                          xe2x80x83                        ⁢            by                                                            i            =                          floor              ⁢                              xe2x80x83                            ⁢                              (                                  t                  /                  T                                )                                                                                                                    C                i                new                            =                              C                i                last                                      ,                          for              ⁢                              xe2x80x83                            ⁢              the              ⁢                              xe2x80x83                            ⁢              other              ⁢                              xe2x80x83                            ⁢              i                                          
Further, synchronously with the medium rotation, a feed-forward output unit reads out the height Ci of each rectangular function of the approximated function TES{circumflex over ( )}repeat(t) stored in the memory cell synchronously with the sampling period Tsample, adds it to the position signal TES from the position signal detecting unit, and supplies a feedback signal TESFB to the feedback calculating unit. In this case as well, the feed-forward output unit reads out the value of the approximated function TES{circumflex over ( )}repeat(t) stored in each memory cell of the memory corresponding to the time that is advanced by the predetermined time xcex94tlead and outputs it.
The learning control unit outputs the approximated function I{circumflex over ( )}repeat(t) or TES{circumflex over ( )}repeat(t) obtained by the learning algorithm after the learning synchronously with the medium rotation, thereby performing a feed-forward control. The learning control unit feed-forward controls such that an operation to obtain an approximated function by the learning algorithm is performed for a specific time at a timing just: after the medium was inserted into the apparatus and, at the time of a track-following control after the learning, the obtained approximated function is outputted synchronously with the medium rotation and the: repetitive disturbance is removed. The learning control unit also feed-forward controls such that, at the time of the track jump and the seek control after the learning, the obtained approximated function is outputted synchronously with the medium rotation and the repetitive disturbance is removed.
In the storage apparatus, when the approximated function is obtained by learning at a specific position in the disk radial direction, for example, at a position near the center region on the disk, in the case where a pickup is sought and moved to another radial direction position and the track-following control is performed, there is a situation such that an error occurs so long as the obtained approximated function is used, so that the approximation is inadequate. For example, when circularity at the inner region of the track on the disk and that at the outer region on the disk are different, when a difference between the phases or amplitudes of the repetitive disturbance in association with the spindle rotation in the inner region and the outer region cannot be ignored, or when a pickup having a structure of a mechanism of the single driving type is used, there is a situation such that magnitudes of friction in the inner region and the outer region differ. Therefore, in the learning control unit of the invention, the getting operation of the approximated function is performed at a plurality of positions in correspondence to the radial direction position of the disk. In the feed-forward mode, the approximated function is selected in accordance with the track address where the pickup is on-tracked at that time (for example, the approximated function obtained by the learning in the nearest track address is selected) and the feed-forward is performed, so that the high precise track-following control can be realized irrespective of the track address to be on-tracked. In the case where the getting operation of the approximated function is performed at a plurality of positions as mentioned above, there is hardly difference among the basic waveforms of the approximated functions and differences among the approximated functions are fine differences. Therefore, when there is approximated function data at another position, an initial value (initial value of the cell corresponding to the height of each rectangular function) of the approximated function data in the approximated function getting operation at the present position is not started from zero but is started by using the approximated function data at another position as an initial value, thereby enabling the learning time to be reduced. In case of the optical storage apparatus, for example, the head has a structure of the single driving type such that the objective lens is mounted onto the carriage that is freely movable in the direction which transverses the tracks on the medium in a manner such that the focusing control can be freely performed, and both the track-following control for allowing the laser beam to trace the track by the movement of the carriage and the seek control for moving the laser beam onto an arbitrary track position are performed.